Spleen cells from mice primed with DNP-BSA produced DNP-specific PFC upon in vitro secondary stimulation with DNP-KLH and propagation in diffusion chambers in the presence of KLH-educated T cells, but not with normal T cells. It was found that the helper activity of specific T cells could be replaced by optimal concentrations of rabbit anti-KLH serum but not by anti-DNP serum. Thus, anti-carrier antibody appears to by-pass the requirement for helper T cells in the secondary response. In other work, tolerance to the thymus-dependent antigen lysozyme and to the thymus-independent pneumococcal polysaccharide type III was terminated by in vitro exposure of spleen cells from the tolerant mice to complexes of tolerogen and specific antibody, but not to the tolerogen alone. The antibodies were xenogeneic, allogeneic and syngeneic to the tolerant mice.